Use of organic materials in electronic devices makes such devices relatively inexpensive, easy process ability and cost effective over inorganic materials. In addition, the inherent property of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, at which an organic emissive layer emits light may generally be readily tuned with appropriate selection of metal ion, ligands modification (introduction of electron donating or electron withdrawing moiety), increasing chain length or by appropriate dopants etc.
The term ‘organic’ includes small molecule organic materials as well as polymeric materials that may be used to fabricate organic electronic device such as OLEDs. Metal complexes having organic ligand part also fall in the category of small molecule organic materials. Small molecules may actually be quite large and may include some repeated unit. For example, central metal having a long chain alkyl group as a substituent in the ligand does not remove a molecule from the ‘small molecule’ class. Such metal complexes may also be incorporated into polymers disclosed in U.S. Pat. No. 5,066,695. Small molecule may also serve as the metal centered core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecular emitter. A dendrimer may be a small molecule and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.
Although lithium metal quinolates are known complexes but so far as the previous methods of preparation of Liq are concerned, the complex is made by reacting the alkyllithiums viz. n-butyllithium, 1-hexyllithium, 2-ethylhexyllithium, 1-octyllithium etc. with 8-hydroxyquinoline in an inert atmosphere. Alkyllithiums are quite difficult to prepare as described in U.S. Pat. Nos. 3,122,592, 3,293,313, 3,420,903, 5,332,533, 5,626,798, 5,663,398, 7,005,083, 20040251562, where lithium metal is dispersed to a particle size of 300 micron, washed with hexane and pentane, drying in argon and then lithium is transferred through a tube to a Morton Cleaved 3-nacked flask equipped with a reflux condenser, Y-tube for addition, pressure-equalizing addition funnel, stirring shaft with teflon blade, stirring motor, a thermometer probe and means for maintaining an inert atmosphere in the reactor. The lithium-hexane mixture is heated to reflux (dry ice/hexane in condenser) and dropwise feed of alkylchloride. The process needs continuous attention and reaction heat is controlled strictly by the rate of reflux. Hence the synthesis of lithium quinolate from alkyl lithium is quite difficult, also the handling of alkyl lithium are quite difficult and reaction of these require special attention
Operating voltage is a key feature of an organic electroluminescence device, in order to reduce the operating voltage of the device, the injection of electrons and holes should be balanced. It is needed to improve electron injection ability. The use of low work function metal as a cathode can improve electron injecting ability. However, the low work function metals are too active and opt to react with oxygen and water. Another method for improving electron injecting ability is to add an electron injecting layer formed of an inorganic compound layer between said cathode and an organic layer as described in U.S. Pat. Nos. 6,172,459, 6,023,073, 6,013,384, 5,989,737 etc. It has been proved in practice that LiF/Al is a cathode structure having excellent electron injecting ability, which is widely used in organic electronic devices. However, the major problem with these materials is that upon slightly increasing the thickness of these materials from required one, they form highly insulating contact. Different metal complexes are also used as electron injecting materials.
In organic electronic devices particularly in OLEDs there are various organic layers are present in between the electrodes. In layers comprising organic materials and metal complexes, the number of charge carriers may be very low, significantly limiting the conductivity of the layer. A particular solution of this problem involves doping the film with redox active dopents, leading to controlled oxidation or reduction of the carrier transporter, creating finite charge carriers. Alq3, phenanthrolines and other electron transporting materials have been successfully doped with Li to significantly enhance their conductivity. [(a) ‘Lithium doping of semiconducting organic charge transport materials.’ Parthasarathy G., Shen C., Kahn A., Forrest S. R. J. Appl. Phys. (2001), 89(9), 4986-4992. (b) ‘Low-voltage inverted transparent vacuum deposited organic light emitting diodes using electrical doping.’ Zhou, X.; Pfeiffer, M.; Huang, J. S.; Blochwitz-Nimoth, J.; Qin, D. S.; Werner, A.; Drechsel, J.; Maennig, B.; Leo, K. Appl. Phys. Lett. (2002), 81(5), 922-924. (c) ‘Efficient multilayer organic light emitting diode’ Liu, Z.; Pinto, J.; Soares, J.; Pereira, E. Synthetic Metals (2001), 122(1), 177-179. (d) Electron structure of tris(8-hydroxyquinoline)aluminum thin film in the pristine and reduced states.’ Johansson, N.; Osada, T.; Stafstrom, S.; Salaneck, W. R.; Parente, V.; Dos Santos, D. A.; Crispin, X.; Bredas, J. L. J. Chem. Phys. (1999), 111(5), 2157-2163. (e) ‘Bright organic electroluminescent devices having a metal doped electron-injecting layer.’ Kido, J.; Matsumoto, T. Appl. Phys. Lett. (1998), 73(20), 2866-2868. (f) ‘Improved efficiency of organic light emitting devices employing bathocuproine doped in the electron-transporting layer.’ Wu, Z.; Yang, H.; Duan, Tu; Xie, W.; Liu, S.; Zhao, Yi. Semicond. Sci. Technol. (2003), 18, L49-L52.]. However, there are two potential drawback of Li doping is that the number of free charge carriers generated by lithium doping is far less then the amount of Li that doped into the film(carriers/Li<10%). The low yield of free carriers is thought to be due to the formation of charge transfer complexes, or tightly bound ion pairs. [‘Investigation of interface formation between calcium and tris(8-hydroxyquinoline)aluminum.’ Choong, V. E.; Mason, M. G.; Tang, C. W.; Gao, Y. Appl. Phys. Lett. (1998), 71(21), 2689-2691]. A second problem of Li doping is that Li may be highly mobile, readily diffuse throughout the device and markedly degrade the device performance due to the formation of trapping or quenching sites. Clearly the problem with Li doping are related to its high charge density and small size.